Axial thrust control for rotary compressors

ABSTRACT

Systems and methods are used to control operation of a rotary compressor of a refrigeration system to limit or prevent movement of rotors due to axial thrust loading resulting from rapid changes in speed of the rotors of the compressor. The operational profile of the motor is controlled to maintain acceleration torque and deceleration torque within predefined limits. The acceleration torque and deceleration torque are maintained within the predefined limits by controlling the speed of the motor, or by controlling the torque applied by the motor to the rotors during acceleration or deceleration.

CROSS-REFERENCE TO RELATED APPLICATION

The present application claims the benefit of Provisional ApplicationSer. No. 61/758,399 filed on Jan. 30, 2013, which is incorporated hereinby reference for all purposes.

BACKGROUND

Compressors in refrigeration systems raise the pressure of a refrigerantfrom an evaporator pressure to a condenser pressure. The evaporatorpressure is sometimes referred to as the suction pressure and thecondenser pressure is sometimes referred to as the discharge pressure.Many types of compressors, including screw compressors and scrollcompressors, are used in such refrigeration systems. Such compressorsare generally referred to herein as rotary compressors.

A rotary compressor includes a suction port and a discharge port thatopen into a working chamber of the compressor. For screw compressors,the working chamber includes a pair of meshed screw rotors that define acompression pocket between the screw rotors and interior walls of theworking chamber. Refrigerant is received by the suction port anddelivered to the compression pocket. Rotation of the rotors closes thecompression pocket from the suction port and decreases the volume of thecompression pocket as the rotors move the refrigerant toward thedischarge port. Due to decreasing the volume of the compression pocket,the rotors deliver the refrigerant to the discharge port at a dischargepressure that is greater than the suction pressure.

For scroll compressors, the working chamber includes two spiral rotorsinterlocking with one another. One of the rotors is fixed and the otheris driven by a motor to rotate relative to and against the sides of theother rotor. This generates suction, creating pockets of refrigerantthat progress from the suction side around the rotors air and compressas they move toward the center of the rotors, where the refrigerant isthen forced to the discharge port.

In addition to these general operational characteristics of rotarycompressors in which the compression and movement of fluid producesaxial thrust forces, rapid changes in rotational velocity of the rotorscan cause axial rotor movement or thrust. If the rotors travel too far,they can contact surfaces of the compressor that were not intended tocontact the rotors. As a result, the potential for failure and excessivewear of these components is created by axial movement of the rotors.While these thrust forces can be counter-acted by bearings, large thrustforces tend to cause wear of the thrust bearings, increase the loadingon the compressor, and reduce reliability of the compressor and bearingsover time. Therefore, further improvements in methods and systems forcontrolling axial thrust forces during operation of rotary compressorsare desirable.

DISCLOSURE

For the purposes of clearly, concisely and exactly describing exemplaryembodiments of the invention, the manner and process of making and usingthe same, and to enable the practice, making and use of the same,reference will now be made to certain exemplary embodiments, includingthose illustrated in the figures, and specific language will be used todescribe the same. It shall nevertheless be understood that nolimitation of the scope of the invention is thereby created, and thatthe invention includes and protects such alterations, modifications, andfurther applications of the exemplary embodiments as would occur to oneskilled in the art to which the invention relates.

SUMMARY

Embodiments of refrigeration systems, compressor systems and methods tocontrol rotary compressors of such systems to limit or avoid axialmovement of the rotors due to axial thrust loads during certainoperating conditions are disclosed. An embodiment of a method and systemincludes controlling operation of a rotary compressor of a refrigerationsystem by controlling the acceleration and/or deceleration torque of therotors according to predefined acceleration/deceleration limits whenchanging the speed or velocity of the compressor. Embodiments of themethod and system include controlling the speed of the motor driving thecompressor to maintain changes in speed of the motor within predefinedacceleration/deceleration limits, and controlling the accelerationtorque and deceleration torque applied by the motor in changing thespeed or velocity of the compressor within predefinedacceleration/deceleration limits. The method and system may also includedriving the rotary compressor within the predefinedacceleration/deceleration limits. Embodiments of refrigeration systemsand/or compressor systems suitable for implementing disclosedembodiments of controlling operation of the rotary compressor are alsopresented. Further embodiments, forms, objects, features, advantages,aspects, and benefits shall become apparent from the followingdescription and figures.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 shows an embodiment of a refrigeration system that includes acompressor system.

FIG. 2 shows additional details of the compressor system of FIG. 1.

FIG. 3 shows a flowchart of a control method implemented by thecompressor system of FIG. 1.

FIG. 4 is a graph of compressor speed commands and a compressor speedprofile to respond to the compressor speed commands that is controlledwith acceleration/deceleration limits to limit axial thrust movement ofthe rotors.

FIG. 5 is a graph of compressor speed commands and a compressor torqueand speed profile to respond to the compressor speed commands controlledwith acceleration/deceleration limits to limit axial thrust movement ofthe rotors.

DETAILED DESCRIPTION

FIG. 1 depicts one embodiment of a refrigeration system 10. Therefrigeration system 10 may circulate a fluid such as, for example, arefrigerant, as indicated by arrows 62, 64, 66 in order to receive acooling load and remove the heat from the load for rejection elsewhere.As shown, the refrigeration system 10 includes a compressor system 12, acondenser system 18 coupled to the compressor system 12, and anevaporator system 20 coupled between the compressor system 12 and thecondenser system 18. Other components and systems may also be provided,such as expansion valves, economizers, pumps, and the like as would beunderstood by those of ordinary skill in the art.

The compressor system 12 may include a suction port 14 and a dischargeport 16. As known to those skilled in the art, the suction port 14 ofcompressor system 12 receives the fluid in a first thermodynamic state,and the compressor system 12 compresses the fluid and transfers thefluid from the suction port 14 to the discharge port 16 at a higherdischarge pressure and a higher discharge temperature. The fluiddischarged from the discharge port 16 may be in a second thermodynamicstate having a temperature and pressure at which the fluid may bereadily condensed with cooling air or cooling liquid in condenser system18. The condenser system 18 receives the compressed fluid from dischargeport 16 of the compressor system 12 and cools the compressed fluid as itpasses through the condenser system 18. The condenser system 18 mayinclude coils or tubes through which the compressed fluid passes andacross which cool air or cool liquid flows to reject heat to the air orother medium.

The evaporator system 20 receives the cooled fluid from the condensersystem 18 after passing through any intervening expansion valve and/oreconomizer and routes the cold fluid through coils or tubes of theevaporator system 20. Warm air or liquid providing a load is circulatedfrom the space to be cooled across the coils or tubes of the evaporatorsystem 20. The warm air or liquid passing across the coils or tubes ofthe evaporator system 20 causes a liquid portion of the cold fluid toevaporate. At the same time, the warm air or liquid passed across thecoils or tubes may be cooled by the fluid, thus lowering the temperatureof the space to be cooled. The evaporator system 20 then delivers theevaporated fluid to the suction port 14 of the compressor system 12 as asaturated vapor. The evaporator system 20 completes the refrigerationcycle and returns the fluid to the compressor system 12 to berecirculated again through the compressor system 12, condenser system18, and evaporator system 20.

Referring to FIG. 2, further details of one embodiment of the compressorsystem 12 are shown. The compressor system 12 may include a controller50, a memory 52 as part of or connected to controller 50, an electricmotor system 30, and a rotary compressor 22. Electric motor system 30 isconnected to rotary compressor 22 with a shaft 32 that drives rotors 24,26 in response to operation of motor system 30. The compressor system 12may further include one or more electrical sensors 31 and torque sensors33 associated with motor system 30 that transmit signals to controller50 via communications links 34 and 35, respectively. Compressor system12 may also include suction pressure and/or temperature sensors 25, anddischarge pressure and/or temperature sensors 27, associated withcompressor 22 that transmit signals to controller 50 via communicationslinks 28 and 29, respectively. The sensors 25, 27, 31, 33 provideoperational parameters 104 (FIG. 3) of the motor system 30 andcompressor 22 with measurements that are indicative of the operation ofthe compressor 22. Furthermore, the sensors 25, 27, 31, 33 can beconnected to controller 50 via a wired connection, wireless connect, andcombinations thereof. In addition, sensors 25, 27, 31, 33 can be virtualsensors.

The controller 50 may include processors, microcontrollers, analogcircuitry, digital circuitry, firmware, and/or software that cooperateto control operation of the motor system 30 and the rotary compressor22. The memory 52 may be a part of controller 50 or a separate device,and comprise non-volatile memory devices such as flash memory devices,read only memory (ROM) devices, electrically erasable/programmable ROMdevices, and/or battery backed random access memory (RAM) devices tostore an array of acceleration/deceleration limits 54 for the operationof motor system 30 and rotary compressor 22. The memory 52 may furtherinclude instructions which the controller 50 may execute in order tocontrol the operation of motor system 30 and the rotary compressor 22.

Some aspects of the described systems and techniques may be implementedin hardware, firmware, software, or any combination thereof. Someaspects of the described systems may also be implemented as instructionsstored on a machine readable medium which may be read and executed byone or more processors. A machine readable medium may include anystorage device to which information may be stored in a form readable bya machine (e.g., a computing device). For example, a machine readablemedium may include read only memory (ROM); random access memory (RAM);magnetic disk storage media; optical storage media; flash memorydevices; and others.

As explained in more detail below, the controller 50 may receiveoperational signals from one or more sensors 25, 27, 31, 33 of thecompressor system 12 that provide information regarding operationparameters of the motor system 30 and the rotary compressor 22. Basedupon the operational signals, the controller 50 may determine anoperating mode of the rotary compressor 22 and may generate, based uponthe determined operating mode and/or operating point, one or morecommand signals 56 that command an operating parameter of motor system30 and/or rotary compressor 22 and an accompanying command signalprofile that controls the operation of the motor system 30 and rotarycompressor 22 over time to meet the command. In particular, thecontroller 50 in one embodiment may select an acceleration/decelerationlimit from the array of acceleration/deceleration limits 54 stored inmemory 52 or may otherwise determine an acceleration/deceleration limitfor the rotary compressor 22 based upon the operating mode determinedfrom the operational signals. The controller 50 may then generatecommand signals 56 that request the electric motor system 30 to delivera particular speed to the rotary compressor 22 via a command signalprofile that is governed by the acceleration/deceleration limit 54selected based on the operating mode of the rotary compressor 22.

The electric motor system 30 drives the rotary compressor 22 in responseto command signals 56 received from the controller 50 according to thecommand signal profile. In the illustrated embodiment, the electricmotor system 30 includes a variable frequency drive 40 and an electricmotor 45. The electric motor 45 is coupled to the rotary compressor 22via shaft 32 to drive rotors 24, 26 of the rotary compressor 22 thatextend axially between ports 14, 16. In one embodiment, the electricmotor 45 includes a permanent magnetic motor that drives the rotors 24,26 at a speed that is dependent upon the frequency of polyphase controlsignals 42 and at a torque that is dependent upon the electric currentsupplied by the polyphase control signals 42. The variable frequencydrive 40 receives command signals 56 from the controller 50 andgenerates and adjusts the polyphase phase control signals 42 accordingto the command signal profile that is governed by the selectedacceleration/deceleration limit 54. In particular, the variablefrequency drive 40 adjusts the frequency and electric current of thepolyphase control signals 42 based upon the command signal profile ofcommand signals 56 received from the controller 50. As mentioned above,the controller 50 may generate the command signal profile that isgoverned by an acceleration/deceleration limit selected for the rotarycompressor 22. As such, the variable frequency drive 40 in response tothe command signal profile adjusts at least one of the frequency andcurrent of the control signals 42 in a manner that satisfies theacceleration/deceleration limit selected for the rotary compressor 22 inorder to meet the command signal 56 over time.

As shown, the electrical sensor 31 is positioned proximate the electricmotor 45 to sense electrical operating characteristics of the electricmotor 45. The electrical sensor 31 may further provide operationalsignals via communications link 34 with measurements that are indicativeof the sensed electrical operating characteristics. In one embodiment,the electrical sensor 31 includes one or more current sensors. Thecurrent sensors may be positioned to sense the electric current suppliedby the control signals 42 to the electric motor 45 and may generateoperational signals that are indicative of the sensed electric current.In one embodiment, the torque produced by the electric motor 45 isdependent upon the electric current supplied by the control signals 42.Accordingly, operational signals indicative of the electric currentsupplied to the electric motor 45 are also be indicative of the torquesupplied by the electric motor 45. While the electrical sensor 31 in oneembodiment comprises current sensors that sense current supplied to theelectric motor 45, the electrical sensor 31 may sense other electricaloperating characteristics of the electric motor 45 such as voltages,currents, phase angles, frequencies, effective impedances at the inputand/or other parts of the electric motor 45 and provide operationalsignals indicative of the sensed electrical operating characteristics.

As shown, the torque sensor 33 may be positioned proximate the electricmotor system 30 to sense torque applied by the electric motor system 30to the rotary compressor 22. The torque sensor 33 may further provideoperational signals with measurements that are indicative of the sensedtorque. In one embodiment, the torque sensor 33 may include one or moretorsion elements positioned between the electric motor 45 and the rotarycompressor 22. The torque sensor 33 may then generate operationalsignals indicative of the torque generated by and/or applied to thetorsion elements. In addition, the suction pressure and/or temperaturesensor 25 are positioned proximate the suction port 14 of the rotarycompressor 22 to sense pressure and/or temperature of the fluid enteringthe suction port 14. Likewise, the discharge pressure and/or temperaturesensor 27 may be positioned proximate the discharge port 16 of therotary compressor 22 to sense pressure and/or temperature of the fluiddischarged from the discharge port 16. The suction pressure and/ortemperature sensors 25, 27 provide operational signals with measurementsthat are indicative of the sensed pressure and/or temperature of thefluid entering the suction port 14 and the discharge port 16,respectively.

In one embodiment, rotary compressor 22 in one embodiment is a screwcompressor that includes a plurality of meshed screw type rotors 24, 26.The meshed screw rotors 24, 26 define one or more compression pocketsbetween the rotors 24, 26 and interior chamber walls of the compressor22. The torque supplied by the electric motor 45 rotates the screwrotors 24, 26, thus closing the compression pocket from the suction port14. Rotation of the rotors 24, 26 further decreases the volume of thecompression pocket as the rotors 24, 26 move the fluid toward thedischarge port 16. Due to decreasing the volume of the compressionpocket, the rotors 24, 26 deliver the fluid to the discharge port 16 ata discharge pressure that is greater than the suction pressure and at adischarge temperature that is greater than the suction temperature. Inanother embodiment, rotary compressor 22 includes scroll-type rotorsthat compress and advance the fluid from suction port 14 to dischargeport 16.

The operation of the rotary compressor 22 in compressing and moving thefluid produces axial thrust forces which may cause axial movement ofrotors 24, 26 against chamber walls, bearings, and end surfaces of thescrew compressor 22. While thrust bearings and lubricating oil providessome resistance and cushioning for the chamber walls, rotors 24, 26, andbearings of the rotary compressor 22, under certain operating conditionsthe axial thrust loads and/or movement may produce undesirable results,such as increased wear, decreased reliability, and potential failure. Inselecting appropriate acceleration/deceleration limits for operation ofthe rotary compressor 22, the controller 50 drives the rotary compressor22 in a manner which reduces or eliminates the non-productive axialthrust forces to at least limit if not prevent axial movement of therotors under the controlled operating conditions. In one embodiment, anallowable axial thrust load on rotors 24, 26 is determined that wouldmaintain axial movement of the rotors 24, 26 within desired limits. Theallowable acceleration torque and the allowable deceleration torque ofrotors 24, 26 is determined based on this allowable axial load. Theacceleration/deceleration limits are based on the allowable accelerationand deceleration torques of the rotors so that axial thrust loads thatcause undesired axial movement of the rotors are prevented.

Due to the unique operating characteristics of rotary compressor 22 atdifferent operating conditions, beneficial acceleration/decelerationlimits 54 can be identified for different operating modes and/or atoperating points in such operating modes. For example, the rotarycompressor 22 may be operated in any one of a start mode, anaccelerating mode, a decelerating mode, and a stop mode, each of whichmay involve different acceleration/deceleration limits 54 to limit axialthrust and prevent undesired movement of compressor 22. An array ofacceleration/deceleration limits 54 for associated operating modesand/or operating points may be established for the rotary compressor 22and stored in memory 52 of controller 50.

In one embodiment, each acceleration/deceleration limit 54 comprisescontrolling the amount of accelerating/decelerating torque applied byshaft 32 of motor 45 to compressor 22 by setting direct limits on theincremental acceleration current and the incremental decelerationcurrent supplied by variable frequency drive 40 to motor 45.Acceleration and/or deceleration torque can be varied up to theassociated acceleration/deceleration limit 54 for the particularoperating mode or point. In another embodiment,acceleration/deceleration limits 54 comprise controlling the speedprofile of the compressor 22 by limiting the rates of change in thespeed of motor 45 via frequency control signals are no more thanseparate limit values for the acceleration/deceleration limit 54 for theparticular operating mode or point. Acceleration and/or decelerationspeed rates of change can be varied up to the associatedacceleration/deceleration limit 54 for the particular operating mode orpoint.

Referring now to FIG. 3, an embodiment of a control method 100 that maybe implemented by the controller 50 for operation of compressor system12 is shown. The controller 50 in one embodiment continuously executesthe control method 100 of FIG. 3 in order to limit accelerationand/deceleration of rotary compressor 22 by operation of motor 45 withvariable frequency drive 40. Control method 100 begins at 102 upon startof compressor system 12. At operation 104, the controller 50 receivesoperational parameters from the signals of various sensors 25, 27, 31,33 of the compressors system 12 that provide information regarding thepresent operation of motor system 30 and rotary compressor 22. Thecontroller 50 at conditional 106 determines whether the compressorsystem 12 is in a start mode based upon the operational parameters. Thecontroller 50 may also determine whether the compressor system 12 is ina start mode based upon other data of the refrigeration system 10. Forexample, the controller 50 may determine that the compressor system 12is in a start mode in response to a signal from a control panel orthermostat (not shown) that indicates the controller 50 is to turn onthe refrigeration system 10 and start the rotary compressor 22. Inresponse to determining that the compressor system 12 is in a startmode, the controller 50 selects at operation 108 a start accelerationlimit from the acceleration/deceleration limits 54 stored in memory 52that governs the compressor signal profile by providing a limit to theacceleration torque of rotary compressor 22 as it increases from a zerospeed to a starting speed.

In response to determining that the compressor system 12 is not in astart mode at conditional 106, the controller 50 determines atconditional 110 whether the rotary compressor 22 is in an acceleratingmode based upon the operational signals, and/or based upon controlcommands indicating that the rotation speed of the rotors 24, 26 is toincrease. In one embodiment, the controller 50 determines whether therotary compressor 22 is to accelerate based upon a comparison of acurrent operating speed of compressor 22 to a compressor speed commandsignal 56 from controller 50. In response to determining thatacceleration of the rotary compressor 22 is required, the controller 50selects at operation 112 a run acceleration limit from theacceleration/deceleration limits 54 stored in memory 52 that governs thecompressor signal profile by providing a limit to the accelerationtorque of rotary compressor 22 from its current operating speed to thespeed required by the compressor speed command signal 56 from controller50.

In response to determining that the rotary compressor 22 is not in astart mode or accelerating mode, the controller 50 at conditional 114determines whether the rotary compressor 22 is in a decelerating modebased upon the operational signals, and/or based upon control commandsindicating that the rotation speed of the rotors 24, 26 is to decrease.In one embodiment, the controller 50 determines whether the rotarycompressor 22 is to decelerate based upon a comparison of a currentoperating speed of compressor 22 to a compressor speed command signal 56from controller 50. In response to determining that deceleration of therotary compressor 22 is required, the controller 50 selects at operation116 a run deceleration limit from the acceleration/deceleration limits54 stored in memory 52 that governs the compressor signal profile byproviding limits to the deceleration torque of rotary compressor 22 fromits current operating speed to the speed required by the compressorspeed command from controller 50.

In response to determining that the rotary compressor 22 is not in astart mode, an accelerating mode, or a decelerating mode, the controller50 at conditional 118 determines whether compressor system 12 is in astop mode. The controller 50 receives operational signals from varioussensors 25, 27, 31, 33 of the compressors system 12 that provideinformation regarding the present operation of motor system 30 androtary compressor 22 and to determine if compressor system 12 is toinitiate a stop mode based upon data supplied by the operationalsignals. The controller 50 may also determine whether the compressorsystem 12 is in a stop mode based upon other data of the refrigerationsystem 10. For example, the controller 50 may determine that thecompressor system 12 is in a stop mode in response to a signal from acontrol panel or thermostat (not shown) that indicates the controller 50is to turn off the refrigeration system 10 and stop the rotarycompressor 22. In response to determining that the compressor system 12is in a stop mode, the controller 50 selects at operation 120 a stopdeceleration limit from the acceleration/deceleration limits 54 storedin memory 52 that governs the compressor signal profile by providinglimits of the deceleration of rotary compressor 22 from its currentspeed to a zero speed.

In response to determining that the compressor system 12 and rotarycompressor 22 are not in a start mode, an accelerating mode, adecelerating mode, or a stop mode, the controller 50 at operation 122selects default acceleration/deceleration limits that are applied duringperiods of relatively stable or steady operation in which the rotationspeed of the rotors 24, 26 is relatively constant. The defaultacceleration/deceleration limits can be provided to limit sudden changesin compressor speed and torque while allowing various threshold levelsto ensure that minor fluctuations in the rotation speed and torque areallowed without a mistaken determination that the rotary compressor 22is operating in an acceleration mode or deceleration mode.

In response to selections 108, 112, 116, 120 of theacceleration/deceleration limits 54 for the particular operating mode106, 110, 114, 118, 122, the controller 50 at operation 124 generates acommand signal profile per the selected limits that limits the changesin speed or the torque of rotary compressor 22 until the compressorspeed required by command signal 56 is met by the actual compressorspeed. Referring to FIG. 4, a graph 200 provides one example ofcompressor operation with commands for controlling the rate of change ofthe speed of compressor 22 over time for the various modes 106, 110,114, 118, 122 by controlling the frequency at which motor 45 operateswith control signals 46 from variable frequency drive 40. The compressorspeed command 202 associated with command signal 56 is represented by asolid line. The compressor speed command 202 includes a start command attime t₀ to increase the compressor speed from zero peed to a first speeds₁. At time t₁, the compressor speed command 202 reduces the compressorspeed from first speed s₁ to a second speed s₂. At time t₃, thecompressor speed command 202 stops operation of compressor 22 byreducing the compressor speed from speed s₂ to a zero speed.

Graph 200 further includes a compressor command signal profile 204 thatis represented by a dashed line. Compressor command signal profile 204controls the changes in compressor speed in a manner that is governed bythe acceleration/deceleration limits 54 selected according to controlmethod 100 to achieve the compressor speed command 202. For example,compressor command signal profile 204 includes a start accelerationprofile 204 a that limits the increase of the speed of compressor 22during starting from a zero speed at time t₀ in a manner that is limitedby the starting acceleration limits 108. Compressor command signalprofile 204 further includes a running acceleration profile 204 b thatincreases the speed of compressor 22 during running of compressor 22with a running acceleration that is limited by the running accelerationlimits 112. The speed of compressor 22 is increased at a rate that islimited by the running acceleration profile 204 b until the compressorspeed reaches speed s₁ of the compressor speed command 202, upon whichcompressor 22 is operated at a steady speed profile 204 c with thedefault acceleration/deceleration limits 122 until time t₁.

At time t₁, compressor command signal profile 204 includes a runningdeceleration profile 204 d that decreases the speed of compressor 22during running of compressor 22 with a running deceleration that islimited by the running deceleration limits 116. The speed of compressor22 is decreased in a manner that is limited by the running decelerationprofile 204 d until the compressor speed reaches speed s₂ of thecompressor speed command 202, upon which compressor 22 is operated atsteady speed profile 204 c at the default acceleration/decelerationlimits 122 until time t₂. At time t₂, compressor speed command 202requires stopping of compressor 22. The rate of deceleration of thespeed of compressor 22 is limited by stopping deceleration limits 120 asrepresented by stopping deceleration profile 204 e until a zerocompressor speed is reached.

Referring to FIG. 5, a graph 300 provides another example of compressoroperation with commands for limiting the amount of torque applied tocompressor 22 over time for the various modes 106, 110, 114, 118, and122 by controlling the current at which motor 45 operates with controlsignals from variable frequency drive 40. The controller compressorspeed command 302 is represented by a line 1 and the compression torque308 is represented by a line 4. The compressor acceleration/decelerationtorque command signal profile 304 is represented by line 2 and islimited by the dashed lines indicating the acceleration torque limit andthe deceleration torque limit. At time t₀, compressor speed command 302indicates a request for an increase in compressor speed. The compressorspeed 306, represented by line 3, is increased by increasing the torqueprovided to compressor 22 via acceleration/deceleration torque commandsignal profile 304 at a rate that is limited by the acceleration torquelimit until the compressor speed 306 reaches the controller compressorspeed command 302. Similarly, a decrease in compressor speed 306 inresponse to compressor speed command 302 at time t₁ or time t₂ resultsin a deceleration torque being applied to compressor 22 via theacceleration/deceleration torque command signal profile 304 limited bythe deceleration torque limit. As shown in FIG. 4 at time t₁, the rateof acceleration or deceleration can be less than the limits. The controlmethod represented by FIG. 4 controls the amount of torque used tochange compressor speed by placing limits on the acceleration torque anddeceleration torque.

It shall be understood that the exemplary embodiments summarized anddescribed in detail above and illustrated in the figures areillustrative and not limiting or restrictive. Only the presentlypreferred embodiments have been shown and described and all changes andmodifications that come within the scope of the invention are to beprotected. It shall be appreciated that the embodiments and formsdescribed below may be combined in certain instances and may beexclusive of one another in other instances. Likewise, it shall beappreciated that the embodiments and forms described below may or maynot be combined with other aspects and features disclosed elsewhereherein. It should be understood that various features and aspects of theembodiments described above may not be necessary and embodiments lackingthe same are also protected. In reading the claims, it is intended thatwhen words such as “a,” “an,” “at least one,” or “at least one portion”are used there is no intention to limit the claim to only one itemunless specifically stated to the contrary in the claim. When thelanguage “at least a portion” and/or “a portion” is used the item caninclude a portion and/or the entire item unless specifically stated tothe contrary.

What is claimed is:
 1. A compressor system, comprising: a rotarycompressor comprising a suction port to receive fluid at a suctionpressure, a plurality of axially extending rotors to compress the fluid,and a discharge port to discharge the compressed fluid at a dischargepressure that is higher than the suction pressure; an electric motorconfigured to receive control signals and drive the plurality of rotorsat a commanded speed determined by the received control signals; acontroller configured to: receive operational signals indicative of anoperating mode of the rotary compressor; select anacceleration/deceleration limit for the rotary compressor based upon theoperating mode of the rotary compressor, wherein theacceleration/deceleration limit is based on acceleration anddeceleration torques of the rotors that limit axial movement of therotors; and generate control signals that drive the electric motor froma current speed to the commanded speed with a command signal profilegoverned by the selected acceleration/deceleration limit; and a variablefrequency drive configured to receive the control signals and to drivethe electric motor according to the command signal profile.
 2. Thecompressor system of claim 1, further comprising a memory connected tothe controller that stores a plurality of acceleration/decelerationlimits, wherein the controller is configured to select theacceleration/deceleration limit from the plurality ofacceleration/deceleration limits based upon the operating mode of therotary compressor.
 3. The compressor system of claim 1, wherein thecommand signal profile changes a speed of the rotors by controlling afrequency of a current from the variable frequency drive to the electricmotor to limit a rate of change in the speed of the electric motoraccording to the selected acceleration/deceleration limit.
 4. Thecompressor system of claim 1, wherein the command signal profile changesan accelerating/decelerating torque of the rotors by controlling anamount of current from the variable frequency drive to the electricmotor to limit the accelerating/decelerating torque of the electricmotor according to the selected acceleration/deceleration limit.
 5. Thecompressor system of claim 1, further comprising a memory connected tothe controller, the memory including a starting acceleration limit, arunning acceleration limit, a running deceleration limit, and a stoppingdeceleration limit, wherein the controller is configured to select theacceleration/deceleration limit from the starting acceleration limit,the running acceleration limit, the running deceleration limit, and thestopping deceleration limit based upon the operating mode of the rotarycompressor.
 6. The compressor system of claim 1, wherein the controlsignals comprise polyphase alternating current control signals, theelectric motor comprises a permanent magnet motor to drive the rotarycompressor at the commanded speed controlled by the polyphasealternating current control signals, and the variable frequency driveadjusts a frequency of the polyphase alternating current control signalsto drive the permanent magnet motor according to the command signalprofile.
 7. The compressor system of claim 1, wherein the controlsignals comprise polyphase alternating current control signals, theelectric motor comprises a permanent magnet motor to drive the rotarycompressor at the commanded speed controlled by the polyphasealternating current control signals, and the variable frequency driveadjusts an amount of the polyphase alternating current driving thepermanent magnet motor according to the command signal profile.
 8. Thecompressor system of claim 1, wherein the rotary compressor is a screwcompressor.
 9. A method to control operation of a rotary compressor of arefrigeration system, comprising: receiving operational signalsregarding operation of the rotary compressor of the refrigerationsystem; determining an operating mode of the rotary compressor basedupon the received operational signals; selecting anacceleration/deceleration limit of the rotary compressor based upon theoperating mode, wherein the acceleration/deceleration limit is based onacceleration and deceleration torques that limit axial movement ofrotors of the rotary compressor; and changing a speed of the rotarycompressor while limiting acceleration/deceleration according theacceleration/deceleration limit.
 10. The method of claim 9, wherein:selecting the acceleration/deceleration limit further comprisesselecting a starting acceleration limit for the rotary compressor inresponse to determining that the rotary compressor is in a start mode;and changing the speed of the rotary compressor includes increasing thespeed of the rotary compressor from a zero speed while limitingacceleration according to the starting acceleration limit.
 11. Themethod of claim 9, wherein: selecting the acceleration/decelerationlimit further comprises selecting a running acceleration limit for therotary compressor in response to determining that the rotary compressoris in an accelerating mode; and changing the speed of the rotarycompressor includes increasing the speed of the compressor from acurrent non-zero speed while limiting acceleration according to therunning acceleration limit.
 12. The method of claim 9, wherein:selecting the acceleration/deceleration limit further comprisesselecting a running deceleration limit for the rotary compressor inresponse to determining that the rotary compressor is in a deceleratingmode; and changing the speed of the rotary compressor includesdecreasing the speed of the compressor from a current non-zero speedwhile limiting deceleration according to the running deceleration limit.13. The method of claim 9, wherein: selecting theacceleration/deceleration limit further comprises selecting a stoppingdeceleration limit for the rotary compressor in response to determiningthat the rotary compressor is in a stop mode; and changing the speed ofthe rotary compressor includes decreasing the speed of the rotarycompressor to a zero speed while limiting deceleration according to thestopping deceleration limit.
 14. The method of claim 9, wherein changingthe speed of the rotary compressor includes controlling a frequency of acurrent to an electric motor driving the rotary compressor to limit arate of change in speed of the electric motor according to the selectedacceleration/deceleration limit.
 15. The method of claim 9, whereinchanging the speed of the rotary compressor includes changing anaccelerating/decelerating torque of the compressor by controlling anamount of current to an electric motor driving the rotary compressorwhile limiting the accelerating/decelerating torque of the electricmotor according to the selected acceleration/deceleration limit.
 16. Arefrigeration system, comprising: a rotary compressor comprising asuction port to receive fluid at a suction pressure, a plurality ofrotors to compress the fluid, and a discharge port to discharge thecompressed fluid at a discharge pressure that is higher than the suctionpressure; a condenser coupled to the discharge port of the rotarycompressor, the condenser being configured to cool and condense fluidreceived from the discharge port; an evaporator coupled to thecondensor, the evaporator being configured to evaporate fluid receivedfrom the condenser to provide fluid to the suction port of the rotarycompressor; an electric motor system configured to receive commandsignals and to drive the plurality of rotors according to the receivedcommand signals; a controller configured to: receive operational signalsindicative of an operating mode of the rotary compressor; select anacceleration/deceleration limit for the rotary compressor based upon theoperating mode of the rotary compressor, wherein theacceleration/deceleration limit is based on acceleration anddeceleration torques of the rotors that limit axial movement of therotors; and generate command signals that limitacceleration/deceleration of the electric motor system according to theselected acceleration/deceleration limit to change a speed of the rotorsfrom a current speed to a commanded speed.
 17. The refrigeration systemof claim 16, further comprising a memory connected to the controller,the memory including a plurality of acceleration/deceleration limits,wherein the controller is configured to select theacceleration/deceleration limit from the plurality ofacceleration/deceleration limits based upon the operating mode of therotary compressor.
 18. The refrigeration system of claim 16, wherein thecontroller is configured to control a frequency of a current to theelectric motor system to limit a rate of the change in speed of therotors according to the selected acceleration/deceleration limit. 19.The refrigeration system of claims 16, wherein the controller isconfigured to control an amount of current to the electric motor systemto limit accelerating/decelerating torque in changing the speed of therotors according to the selected acceleration/deceleration limit. 20.The refrigeration system of claim 16, wherein the rotary compressor is ascrew compressor.